It is well known that water insoluble metal soaps, namely salts of metals with saturated aliphatic acids containing three or more carbons, can be produced by various processes. The most important process, presumably, uses a double composition reaction. In this reaction, a previously prepared aqueous solution of an alkali metal or ammonium salt of the aliphatic acid is reacted with a metal salt to produce the desired metal soap. Metal sulfate, chloride, nitrate and acetate salts are representative of the metal salts which can be used in the process. Even though large amounts of metal soaps are still being produced by this process, the process has substantial disadvantages. The main disadvantage is that the metal soaps obtained are generally insufficiently pure primarily because inorganic salts, such as, for example, sodium or ammonium sulfate or chloride, are carried along and trapped in the precipitating metal soaps.
A so-called direct process for producing metal soaps is described in German Pat. No. 860,210. In this process, a dispersion of a metal oxide or metal hydroxide is directly contacted and reacted with an emulsion of the aliphatic acid in water. This process, however, is only useful with metal oxides which are readily, or relatively readily, soluble in water and thus form the corresponding hydroxides. Metals which form water insoluble hydroxides do not react, even when dispersed, with the free aliphatic acids.
It is also known from German published patent application AS No. 11 89 973 to produce a metal soap by first forming an aqueous suspension of an insoluble metal oxide or hydroxide, introducing carbon dioxide into the suspension to form a basic metal carbonate, and then combining this aqueous suspension with a free aliphatic acid aqueous emulsion. The resulting reaction proceeds smoothly to yield the desired metal soap, as for example zinc soap. It is advantageous for the aliphatic acid aqueous emulsion to be saponified with ammonia and also for the suspension of the basic metal carbonate, such as zinc carbonate, to be made ammoniacal. The precipitated metal soap can be filtered off and, without washing, be dried in the usual manner. Whatever residual ammonia is possibly present is removed with the water.
The described direct process has an advantage over the double decomposition process in that an electrolyte-free product is formed in a single processing step. Also, no electrolyte-containing waste water is obtained since no prior step uses an alkali metal hydroxide and, consequently, environmental pollution is avoided. Nevertheless, there are distinct disadvantages with this process. Thus, it is a batch, rather than a continuous process. Furthermore, special filters, for example, chamber filter presses, are required to separate the precipitated metal soap from the water before the soap is dried in a conventional manner, such as in a flowing air, or circulating air, dryer. An improved process which is not limited to batch operation and/or a filtration of the soap, is accordingly needed.